New Horizons image of the planet Neptune. Its largest moon, Triton, can be seen be seen just off to one side. The spacecraft was almost 3.5 billion kilometres away from the pair when it took this image!

New Horizons spots Neptune and its moon Triton

Triton orbits Neptune the “wrong way”

Triton is often considered to be a twin of Pluto

NASA’s Pluto-bound New Horizons spacecraft—now a little over halfway there—has turned its attention to the planet Neptune and its largest moon, Triton.

Mission controllers periodically test the spacecraft’s cameras by aiming them at other Solar System bodies.

New Horizons’ Long Range Reconnaissance Imager (LORRI) snapped several images of Neptune during the latest annual systems checkout, which ended July 30. Neptune was 23.2 astronomical units (about 3.48 billion kilometres!) from New Horizons when LORRI took aim at the gas giant planet—and Triton made a cameo appearance.

Because Neptune and Triton were so far away, they are hard to tell apart in the images. But Triton can be seen as a dot or blob just off to one side.

New Horizons spacecraft prior to launch in 2006.

“That we were able to see Triton so close to Neptune, which is approximately 100 times brighter, shows us that the camera is working exactly as designed,” says New Horizons Project Scientist Hal Weaver, of the Johns Hopkins Applied Physics Laboratory. “This was a good test for LORRI.”

“As New Horizons has travelled outward across the Solar System, we’ve been using our imagers to make just such special-purpose studies of the giant planets and their moons because this is a small but completely unique contribution that New Horizons can make—because of our position out among the giant planets,” says New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute.

Pluto’s twin: an enigma

Triton was discovered on October 10, 1846 by English astronomer William Lassell, just 17 days after Neptune itself had been discovered (by German astronomer Johann Gottfried Galle).

Dark streaks show where Triton's ice geysers have been active.

Neptune’s big moon is very unusual, in that it is the only large moon that goes around its planet backwards. That is, Neptune rotates from west to east (as does Earth), but Triton orbits in the planet from east to west (unlike our Moon). This is called a retrograde orbit.

The only plausible explanation is that Triton’s didn’t form along with Neptune, but rather was captured as it wandered past. Given that it is almost a twin of Pluto, it is supposed by most astronomers that Triton was a member of the Kuiper Belt—the swarm of small icy worlds that orbit the Sun beyond Neptune.

Triton also is quite big—its diameter of 2,700 kilometres makes it the seventh largest moon in the Solar System.

Its surface is a frozen crust of mostly nitrogen, underneath which is a core thought to be composed of rock and metals and making up two-thirds the moon’s mass.

One of the amazing things about Triton is that it has active geological features. When NASA’s Voyager 2 probe flew past in 1989, it spotted dark geysers shooting up from the surface, and dark streaks on the surface downwind of the geysers. The only other Solar System bodies confirmed to have volcanic activity are Earth, Jupiter’s moon Io, and Saturn’s moon Enceladus.

Unlike many moons—which are covered in craters—Triton has few impact craters. Scientists put this down to the geological activity, such as tectonic processes and volcanoes, which can reshape the landscape and wipe out any traces of craters. But unlike the volcanoes on Earth, on this frozen world the lava consists of water and ammonia!

Voyager 2 also sensed a thin atmosphere as it went past—observations made from Earth in 1990s indicated that the atmosphere was, at that time, thicker than when Voyager was there.

Pluto, here we come!

New Horizons was launched on January 19, 2006, on a trajectory and with a velocity that to reach Pluto in the minimum possible time. In consequence, New Horizons is the fastest spacecraft to leave Earth, having reached a velocity of 58,536 km/h after launch.

The spacecraft reached the orbit of Jupiter in February 2007, passed the orbit of Saturn in June 2008, and is not far away from the orbital distance of Uranus.

New Horizons is due to reach Pluto on July 14, 2015, and conduct a fly-by. It is not equipped with a rocket system to slow down and go into orbit around Pluto; instead, it will go sailing past.

But for around 200 days leading up to the encounter, it will start taking images that are better than best images we currently have of the icy world, so there will be plenty of time to make new discoveries.

Following the encounter, New Horizons will continue into deep space. There is a strong chance that mission controllers will be able to target the spacecraft to do a subsequent fly-by of one of the other icy worlds that inhabit the Kuiper Belt.

Adapted from information issued by NASA / JHU APL.

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